In the following, reference will be made i.a. to the list of prior-art references Ref. 1-12 on pages 20 and 21. These references are hereby incorporated by reference.
Micromachining allows the manufacturing of e g sensors and actuators with dimensions of nanometers to centimeters. Specific examples of micromachined objects include motors, pumps, accelerometers, pressure sensors, chemical sensors, valves, micro-motion systems and grippers. Overall surveys of micromachining are found e.g. in Ref. 1, 2 and 3.
Micromachined structures can be divided into the following three general classes C1-C3:
C1: Micromachined structures without any movable parts (wholly attached), such as holes etched in silicon substrates for fuel injection nozzles, or channels etched in glass or silicon for applications such as electrophoresis and other types of chemical analysis. PA1 C2: Micromachined structures with bending parts (partly freed), such as probe tips for atomic force microscopy and related microscopies, pressure sensors, accelerometers, cantilevered beams, and peristaltic pumps. As an analogy, such a structure may be like a diving board, having one side firmly attached to the ground beside the swimming pool and the other side extending over the water to bend. PA1 C3: Micromachined structures with movable parts (completely freed), such as micromotors and comb drives. As an analogy, such a structure may be like a turntable, the spinning platter on which the record is placed corresponding to the completely freed part, whereas the part which attaches the platter to the turntable corresponds to another part of the structure attaching the freed part to the substrate.
The following three general classes of micromachining are known in the art: (1) surface micromachining, (2) bulk micromachining, and (3) LIGA and variations thereof. The fabrication sequence for the manufacture of a micromachine may include a combination of these techniques.
According to the first one of said techniques of micromachining--surface micromachining--layers are deposited and etched on one side of a wafer as disclosed in Ref. 4. Ref. 5 discloses an example of an organic microactuator manufactured according to this technique. Prior-art surface micromachining requires the selective etching of a temporary sacrificial layer in order to release a device or structure which has been patterned from overlying structural layers. Many combinations of sacrificial/structural layers are possible, as long as the underlying sacrificial layer can be preferentially removed. Sacrificial layers can be manufactured from diverse materials, typically SiO.sub.2 or phosphosilicate glass, but aluminum, photoresist, polyimide, porous silicon and others may also be used.
In contrast thereto, according to the second one of said techniques of micromachining--bulk micromachining--devices or structures are made directly from a silicon wafer or a wafers of another substrate material, such as gallium arsenide (GaAs), by selectively removing unwanted parts from the front and/or back sides of the wafer by an etching process (Ref. 6). Bulk micromachining has been used for the manufacturing of micromachines such as membranes, pumps and accelerometers. Two or more substrates may also be bonded together and etched.
Surface and bulk micromachining methods are compared in Ref. 1 and Ref. 7.
The third one of said techniques of micromachining--LIGA and variations thereof comprises the steps of patterning a photoresist or some other suitable polymer using x-rays or ultraviolet light, and depositing material into the resulting holes in the resist by electroplating. This method, as such, results in loose parts, and movable machines built from such loose parts must therefore be assembled by hand. LIGA can also be combined with the sacrificial layer method (Ref. 8).
The only prior-art technique used for releasing movable parts of micromachined structures is the above-described sacrificial-layer technique. This prior-art method traditionally comprises the step of liberating mechanical structures by underetching another underlying thin sacrificial layer, in order to allow components free rotation, translation, or bending. Drawbacks of this method are mentioned i.a. in Ref. 9 and 10 and will be discussed in detail below. The attached FIGS. 1-5, each of which comprises a cross sectional view (a) and a corresponding top view (b), illustrate the process steps according to a typical example of the prior-art sacrifical-layer technique. FIGS. 1-5 are schematic and are not to scale, and the dimensions in the vertical direction are greatly exaggerated.
In a first step (FIG. 1), a substrate 10, such as a silicon wafer, (optionally presenting pre-deposited layers and/or devices) is covered by a sacrificial layer 12. In a second step (FIG. 2), the sacrificial layer 12 is patterned so that a remaining portion thereof only partly covers the surface of the substrate 10 leaving an exposed surface portion 11 of the substrate 10. In a third step (FIG. 3), a structural layer 14 is deposited over the exposed surface portion 11 of the substrate 10 and over the patterned sacrificial layer 12. In a fourth step (FIG. 4), the structural layer 14 is patterned so that a remaining portion thereof partly covers the exposed surface portion 11 of the substrate 10 and partly cover the patterned sacrificial layer 12. In a fifth step (FIG. 5), the patterned sacrificial layer 12 is removed by an underetching process, whereby the remaining portion of the structural layer 14 presents one attached or fixed part 14a and one free-standing part 14b. The free-standing part 14b may form a movable part of the micromachined structure.
Optionally, multiple levels of patterned sacrificial layers and structural layers may be provided. Subsequent to the application of such multiple structural layers, the sacrificial layers are removed by underetching, thereby freeing overlying parts of the micromachined structure.
Although the sacrificial-layer method illustrated in FIGS. 1-5 works satisfactorily for many applications, to produce free-standing or movable parts of a micromachined structure, it has at least the following four main drawbacks.
A first drawback of the sacrificial-layer method is that the etching agents used to remove the sacrificial layer(s) may also damage the overlying structure. This damage can arise because the etching agents may react chemically with the overlaying structural layers, e.g. dissolving them or oxidizing them. The etching process may also damage the structure by generating heat and bubbles. The heat may damage the materials or may warp the structure, and the gas bubbles may impart excess mechanical stress to the structure.
A second drawback of the sacrificial-layer method is that underetching large structures is time-consuming and may be difficult to complete. Consider as an example a plate of 1000.times.1000 .mu.m.sup.2 lying over a 1 .mu.m thick sacrificial layer to be removed at an etch rate of 0,1 .mu.m/min. The etching agent removes material starting from the edges of the plate and works its way to the center thereof, a distance of 500 .mu.m from the plate edge. Thus, the etching would take at least 5000 min. to complete, i.e. more than three days assuming a constant etch rate. However, the removal of material from deep underneath such a relatively large plate is difficult, because fresh etching agent (liquid or gas) does not circulate freely in confined geometries. Therefore, the etch rate actually decreases over time. As a result of the above, large solid plates are difficult or impossible to release by using this prior-art method, and they are today provided with through-holes, so that the underetching is practical.
A third drawback of the sacrificial-layer method is that the use of a sacrificial layer results in a geometrical step for the structural layer. The sacrificial layer cannot be too thin or the underetching becomes even more difficult. Mechanical stress arising at said step of the structural layer may be a problem. In addition, it is difficult to coat "vertical" walls using micromachining. Therefore, if the structural layer is too thin, it may not connect over said step, making the fabrication impossible.
A fourth drawback is that the structural layer must be self-supporting, because a gap is present between the freed structural part and the substrate after removing the sacrificial layer. Also for this reason, the structural layer cannot be too thin, and this limits what can be manufactured.
U.S. Pat. No. 5,447,600 (fled on Mar. 21, 1994, issued on Sep. 5, 1995) discloses a method for reducing sticking between contacting elements in a micromechanical device. A protective layer is formed on a first contact element to reduce the likelihood of a first contacting element sticking to another contacting element. However, the protective layer is not used for the actual manufacturing of the movable part of the structure or the release thereof from the substrate after manufacturing.